ch5 lan switching--it classroom

7/28/2019 Ch5 LAN Switching--IT Classroom

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CHAPTER 5

LAN SWITCHING

In this chapter we cover the basic principles of LAN switching and bridging that form the basisof the experiments in Lab 5.

The chapter has four sections. Each section covers material that you need to run the lab exercises.

The first section gives an overview of the different types of interconnection devices anddiscusses the differences between them. Section 2 goes into an in depth discussion of LANswitching outlining the fundamentals of backwards learning algorithm and how it is used for

routing. In Section 3 we describe the spanning tree algorithm that is used by transparent bridges

to maintain connectivity and avoid forming loops. Section 4 presents the commands used to

configure the hosts and the routers as bridges.


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TABLE OF CONTENT

1 INTERCONNECTION DEVICES.................................................................................................... 3

1.1 PHYSICAL LAYERDEVICES .......................................................................................................... 31.2 DATA LINKLAYERDEVICES ........................................................................................................ 51.3 IPLAYERDEVICES....................................................................................................................... 61.4 ACOMPARISON BETWEEN THE DIFFERENT INTERCONNECTION DEVICES..................................... 7

2 BRIDGES/LAN SWITCHES............................................................................................................. 8

2.1 TRANSPARENT BRIDGES/LANSWITCHES .................................................................................... 82.1.1 Backwards Learning Algorithm............................................................................................ 102.1.2 Spanning Tree Algorithm...................................................................................................... 13

2.1.2.1 Configuration BPDUs ................................................................................................................ 142.1.2.2 Steps of the Spanning Tree Algorithm....................................................................................... 15

3 TOOLS AND UTILITIES................................................................................................................ 18

3.1 CONFIGURING A PC AS A BRIDGE USING THE GBRCTL UTILITY .................................................. 183.2 CONFIGURING A CISCO ROUTER AS A LANSWITCH .................................................................. 23


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1 Interconnection devices

Interconnection devices are used to interconnect networks at the different layers of the

network architecture. The devices can operate at:

the physical layer such as optical repeaters, hubs, digital cross connects, etc.

the data link layer such as LAN switches/bridges, frame relay switches, etc., the network layer such as a router or a gateway

the transport layer for TCP segment switching

the application layer for overlay networks such as content delivery networksThe various interconnection devices, as shown in Figure 1, play different roles in a

network infrastructure and as such have very different functionalities.

Figure 1. Example of different interconnection devices

1.1 Physical Layer DevicesPhysical layer devices are used to increase the reach or geographic span of a physical

network. As a signal propagates through a medium such as a coaxial cable, a twisted pair,or a fiber, it suffers from attenuation, i.e., signal strength loss. As shown in Figure 2a,

beyond a certain distance, a signal has dropped below a strength threshold that makes it

impossible to recover the information. A physical layer device such as a repeateris used

to amplify the signal before it drops below the threshold (see Figure 1b). The repeaterdoes notprocess the content of the bits in anyway.

Figure 2a Attenuation and Signal Strength

Ethernet

Router

Ethernet

Ethernet

Gateway

Bridge

Repeater

Autonomous

System

Autonomous

System


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Figure 2b. Amplification of a Signal using a Repeater

An Ethernet hub is another example of physical layer device, it serves solely as a conduitfor passing packets from its input interfaces to its output interfaces. It broadcasts all

information that it receives on its input ports to all of its output ports. It does not store

any frames, using cut through switching technology for forwarding the frames, the bits in

a frames header are directly routed to all output ports without waiting for the remainderof the frame to be completely received at the input port. All links that connect devices,

such are hosts and routers, to hubs, come in pairs, e.g., 10BaseT, 100BaseT, one pair is

used for upstream traffic and the second pair is used for downstream traffic. Any data thatis transmitted by a host on the uplink pair is looped back on the downstream pair, re-

creating the collision environment of Ethernet. A host therefore hears its own

transmission. At the same time if another host transmits a frame on its upstream link, itsbits will be broadcast to every port, these will be combined with the loopback

transmissions on other downstream links, creating a collision. A host must therefore

sense the downlink stream before commencing a transmission. But, as in any CSMAenvironment, collisions cannot be avoided since two devices could start their

transmissions at the same time.

Most hubs nowadays include some buffering on the interfaces to minimize collisions.

Dual speed hubs isolate the traffic between the two different speed environments. In otherwords, a 10/100 hub will not broadcast the 10Mbps traffic on the 100Mbps links unless

addressed to a 100Mbps MAC address and vice versa, no 100Mbps traffic is broadcast onthe 10Mbp links unless explicitly addressed to a 10Mbps MAC.

Repeater

Hub

Hub

Hub

DataGeneral

DataGeneral

DataGeneral

DataGeneralDataGeneral

DataGeneral

DataGeneral

DataGeneral


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Figure 2. A Hub Architecture

1.2 Data Link Layer DevicesBridges andLAN switches are devices that operate at the data link layer. Theyinterconnect two or more LANs. A bridge was originally designed to provide a bridge

between two different LAN technologies, e.g., an Ethernet LAN and a token ring LAN.

However, over the past decade Ethernet has become the dominant LAN technology andwe have seen the gradual demise of token ring LANs. Bridges evolved to not only bridge

between two different protocols but to provide another option to hubs and repeaters for

extending the size of an Ethernet network domain. Bridges are intelligent devices, that,contrary to hubs, isolate LAN segments thereby limiting the collision environments and

improving the overall throughput. By isolating LAN segments, one inherently obtains a

more secure network in which data from one segment is not broadcast to another.

A bridge is a store and forward device. Every frame is fully received before forwarding.

Transmission on any outgoing link will only take place one frame at a time. Bridges

cannot prevent collisions from occurring on an Ethernet segment, but they will not relaycollided frames. Similar to hubs, current bridge designs provide dual speed ports,

allowing a mix of 10Mbps and 100Mbps devices to be interconnected.

Figure 4a. Bridge Interconnection Device

LAN switches are multi port (more than 4 port) bridges. LAN switches are touted by

manufacturers as high throughput multi interface devices that can interconnect ports at avariety of speeds, e.g., 10M, 100M, 1G and 10Gpbs. They are also able to operate the

Bridge

IP

802.3

MAC

802.3

MAC802.3

MAC

IP

802.3

MACLAN LAN

Bridge


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links in full duplex mode if directly connected to a network device1. To increase their

speed of operation, LAN switches, like hubs, use cut through switching. Once thedestination address has been processed the packet is forwarded to the appropriate output

port where transmission can be commenced if the link is idle.

Figure 4b. A LAN switch architecture

1.3 IP Layer DevicesDevices that process IP datagrams are considered to be IP layer devices. Routers and

gateways are both examples of an IP layer device. They each forward and route IP

datagrams between different subnets as explained in detail in Chapter 3. The maindistinction between a router and a gateway lies in the functionality of the device vis a vis

an Autonomous System (AS). A router generally operates within an AS whereas agateway operates as a bridge between two ASs. A gateway therefore must run two

routing protocols, IGP internal to its AS and EGP external to its AS with correspondinggateways in other ASs.

1The loopback feature is disabled, the link is treated as a point to point link with no

collisions possible as frames are buffered if other transmissions are in progress.

D ataG en er al

DataGeneral

DataGeneral

DataGeneral

D ataG en er al

DataGeneral

Servers

LAN SwitchHub

Hub

Hosts

Hosts


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Figure 5a. A Router interconnecting 3 subnets

Figure 5b. Operation of a Gateway

1.4 A Comparison between the Different Interconnection DevicesIn the Table 1 we summarize the different features of the devices. Each interconnection

device has a role to play in an enterprise network. The choice of which device to use

depends very much on the needs of the network infrastructure. Hubs are cheap plug andplay devices that provide the most basic connectivity. Bridges serve to isolate segments

and are relatively cheap too. They have more intelligence built into them and similar to

hubs and LAN switches are plug and play devices. LAN switches provide high

throughputs at convenient prices when compared to a router. A router serves to create

subnetworks which provide autonomy to different departments in a campus network.They are not plug and play, requiring a system administrator to setup subnets and re-

configure host network interfaces, but they do give much more flexibility when it comesto network design. Although layer two devices provide dynamic routing, only one route

exists between any two devices. In a router configuration, several routes can exist

between a source and a sink.

Subnet-

workRouter

Subnet-

workRouter

Subnet-

work

Application

TCP

IP

Network

Access

Application

TCP

IP

Network

Access

IP protocol IP protocol

Data

LinkNetworkAccess

IP

NetworkAccess

NetworkAccess

IP

NetworkAccess

Data

Link

Data

Link

IP protocol

RouterRouter HostHost

AS 3

Gateways Gateways

AS 2AS 1


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Features/Device Hubs Bridges LAN Switches Routers

Cost Low Low Low/Medium Medium/High

Ease of

deployment

Plug and Play Plug and Play Plug and Play Requires

system admin.

Dynamic No Yes Yes Yes

TrafficIsolation

None Yes Yes Yes

Autonomy No No No Yes

Table 1. Comparison of the Features of Interconnection Devices

2 Bridges/LAN Switches

LAN switches2

are classified by the forwarding procedure that they use to find and reach

a destination in a meshed network topology. Three different approaches were identified:

Fixed paths

Source routing

Combination of Backwards Learning and Spanning Tree AlgorithmsFixed routing is never the favored choice because it requires system administrator

intervention to create the paths and maintain the network under failures. The sourcerouting approach was very popular in token ring environments. A source would send out

a search packet looking for a destination MAC address. Every bridge the frame reached

would insert its ID/address and then forward the frame to every outgoing port (except theincoming port). The first frame to reach the destination would then be used to send back a

response following the path that the frame took to reach the destination. Every frame

henceforth between that source destination pair carried the full path to reach the end point.Alas, token rings have lost favor in the LAN market, giving way to the ever popular

Ethernet LAN and thus the demise of source routing in interconnected LAN settings.

The third choice is the dominant approach. Bridges that use backwards learning and

spanning tree are referred to as transparentbridges due to their truly plug and play nature.

We discuss both algorithms in detail explaining their operation and how they forward

frames transparently, i.e., hosts and routers are oblivious to their presence in the network.

2.1 Transparent Bridges/LAN SwitchesThe forwarding operation of bridges or LAN switches consists of asking the following

question:

Do I know which port to forward a received frame to?The answer to that question, as illustrated in Figure 6 below for a frame arriving at Port x,

is either YES or NO. YES -> forward the frame to the appropriate port

NO -> flood the frame to all outgoing ports except the incoming port

2We will use the term bridge and LAN switch interchangeably for the remainder of the

discussion in this Chapter.


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Figure 6. Forwarding process in a Bridge

AYES answer is hinged upon finding the destination address in a table. The forwarding

table is created using the backwards learning algorithm, a mechanism by which the

bridge learns how to associate destination MAC addresses with outgoing port numbers.

The bridges must reside in a loop free topology because they use flooding to find a

destination. In Figure 7 below we show an example of a typical meshed bridged network.

Meshed networks are popular for they can tolerate a certain number of link and devicefailures without creating a disconnected segment. As illustrated in the figure, unless aloop free topology is used, the controlled flooding algorithm will not terminate. Bridges

only look at a frames destination and source address and the incoming port. They do not

keep a history of flooded frames3. A loop free topology is created by the bridges using

the spanning tree algorithm discussed in Section 2.1.2 below. The topology is maintained

by constantly monitoring the health of the bridges and the connecting links. Upon

detection of a failure, the bridges will self configure to create a new fully connected loopfree topology. Some failures may result in disconnected parts that will require system

administrator intervention.

3Note that there are no sequence numbers in Ethernet frames and as such it is impossible

for a bridge to maintain a history of previously seen frames unless it maintains a record of

the destination addresses of flooded frames. Bridges do not do that, it would complicate

their otherwise very elegant and simple design.

Bridge 2Port A Port C

Port x

Port B

Forward the frame on the

appropriate port

Is MAC address ofdestination in forwarding

database for ports A, B, or C ?

Flood the frame,i.e.,

send the frame on all

ports except port x.

Found?Not

found ?


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Figure 6. A meshed bridge topology

2.1.1 Backwards Learning AlgorithmThe concept of backwards learning is very simple: Learn about an (source) address from

the direction from which it came, then place that address in a table and use it for

(destination) forwarding. Bridges operate in promiscuous mode, they listen to all trafficthat is broadcast on every link connected to its active ports. By examining the source

MAC address of every packet traversing the link associated with a particular port on the

bridge, the bridge learns what addresses are reachable via that particular port. These

addresses are stored in a forwarding database or table.

Every LAN switch maintains a forwarding database or table. This table contains the

following fields:

MAC address

Outgoing Port number

Timer indicating age of entryThe MAC address refers to the destination address in the MAC frame. The outgoing port

number refers to the port that needs to be used to transmit the frame for that particular

LAN 2

Bridge 2

LAN 5

LAN 3

LAN 1

LAN 4

Bridge 5

Bridge 4Bridge 3

d

Bridge 1


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MAC address. The timer is used to control the age of the entries. When a timer expires,

the entry is deleted from the table. Ever MAC address hit refreshes the timer of that MACaddress entry. The table can be interpreted as follows:

A machine with MAC address lies in direction ofoutgoing port number.

The entry is timer time units old.

If a bridge sees a frame with a destination address that matches one of the entries in its

forwarding table, it will copy the packet into its buffer and forward the packet to thenecessary port. If the outgoing port is the same as the incoming port, it discards the frame.

If the bridge sees a frame for which it has no entry in its forwarding table, it will make

multiple copies of the frame and broadcast it on every outgoing port (excluding the porton which the frame arrived). As the bridges are connected in a loop free tree topology,

the flooding will terminate at the leaves of the tree. Below, in Figure 8, we illustrate the

operation of the backwards learning algorithm by stepping through an example of a frametransmission through a single LAN switch with an initially empty forwarding table.

Figure 8a. Frame arrives on Port 3 Source = x, Destination = y

Figure 8b. First entry in table: Bridge learns that MAC address x is associated withport 3

Port 1

Port 2

Port 3

Port 4

Port 5

Port 6Src=x, Dest=ySrc=y, Dest=xSrc=x, Dest=y

Forwarding Table

Port 1

Port 2

Port 3

Port 4

Port 5

Port 6Src=x, Dest=ySrc=y, Dest=xSrc=x, Dest=y

Forwarding Table

x is at Port 3


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Figure 8c. Bridge does not find entry for y, floods frame on all ports except 3

Figure 8d. Frame from y arrives on port 4. Bridge adds entry for MAC address y andforwards frame to port 3 using entry for MAC address x in its table.

If we now take an example of two bridges and observe the process by which a theforwarding table is filled, we will understand the backwards learning algorithm and how

it is used by the bridges in promiscuous mode. In Figure 9 we show a sample network

with two bridges. Host A initially sends a frame to host F. This is followed by a framefrom host C to host A and then a third frame from host E to host C.

Figure 9. Forwarding Example

Bridge 1 will receive the frame on port 1. With its forwarding table empty, Bridge 1 will

flood the frame on outging port 2. Bridge 2 receives the frame on port 1, it too does not

find an entry in its table and proceeds to flood the frame on outgoing port 2. Destination

Port 1

Port 2

Port 3

Port 4

Port 5

Port 6

Src=x, Dest=ySrc=x, Dest=y

Src=x, Dest=y

Src=x, Dest=y

Src=x, Dest=y

Src=x, Dest=y

x is at Port 3

Src=y, Dest=x

Src=y, Dest=xSrc=x, Dest=y

Src=x, Dest=y

Forwarding Table

Port 1

Port 2

Port 3

Port 4

Port 5

Port 6

Src=x, Dest=y

Src=x, Dest=y

x is at Port 3

Src=y, Dest=x

Src=y, Dest=x

Src Dest

Forwarding Table

y is at Port 4

Bridge 1

Port1

LAN 1

A

LAN 2

CB D

LAN 3

E F

Port2

Bridge 2

Port1Port2


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F finally receives the frame. During this flooding process, both Bridges 1 and 2 learnt

that MAC address A is associated with port 1 on their respective bridges. The secondframe from host C to host A will cause no flooding as both bridges have an entry for

MAC address A. Bridge 2 will ignore the frame as its association for MAC address A is

with port 1 on which it received the frame. But before discarding the frame it will make

an entry in its forwarding table for MAC address C. Bridge 1 will receive the frame onport 2 and forward the frame to port 1 based upon the entry for MAC address A in its

forwarding table. It too will make a new entry in the forwarding table for MAC address C.

The third frame from host E to host C will not cause flooding either as both bridges havenow an entry for MAC address C. Bridge 2 will forward the frame from port 2 to port 1

and at the same time enter MAC address E in the forwarding table. Bridge 1 will ignore

the frame as the outgoing port is the same as the received port. It too will make a newentry in the forwarding table for MAC address E. Below we show the resulting

forwarding tables (ignoring the timer field).

Bridge 1 Bridge 2

MAC Address Port MAC Address PortA 1 A 1

C 2 C 1

E 2 E 2

Table 2. Bridge Forwarding Tables for Example shown in Figure 9

2.1.2 Spanning Tree AlgorithmThe spanning tree algorithm [PERL] is the mechanism by which the LAN switches create

a loop free tree topology. As explained above, meshed topologies are the preferred design

choice in an institutional network to tolerate link and device failures. Floodingmechanisms do not perform well in mesh topologies unless the nodes track the flooded

frames and stop flooding when it is recognized that a frame has already been flooded.Bridges do not track frames and so require to operate in a loop free topology. So long as

the destination does send a response back to the source, the bridges will never find the

destination, even though the frame will have reached the destination. If a loop exists, theframe will be flooded over and over, with each reception of the frame at a bridge

generating a new flood. In other words, we will observe an exponential growth in the

number of flooded frames.

The idea behind the spanning tree is very simple. Create a tree in which some bridges

and/or ports on bridges are active and others are blocked. The blocked bridges and/orports constitute the disconnected portions of the original meshed topology to create the

tree topology.

The spanning tree algorithm uses a specific frame called the Bridge Protocol Data Unit(BPDU) for exchanging information between the bridges. The BPDUs come in various

types. Configuration BPDUs are used to create the tree by exchanging path cost

information, bridge IDs, etc. Hello BPDUs are used to monitor the health of the tree. If atany point a bridge in the tree does not send a hello BPDU within a specified interval of

time, the neighboring bridges (including blocked ones) will sound the alarm by initiating


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a new round for creating a spanning tree. The health monitoring BPDUs are triggered by

the root bridge, the bridge at the root of the spanning tree. The root bridge periodicallybroadcasts a hello BPDU on its branches. The reception of this broadcast hello BPDU by

bridges at the next level on the tree, in turn, triggers a transmission of a hello BPDU

along their branches. This continues down the tree till it reaches the leaf bridge which

transmits a hello BPDU on its local LAN segments primarily for the benefit of anyblocked bridges that might be attached at that level signaling its continued health.

The tree creation process consists of the exchange of configuration BPDUs that informother bridges of the ID of the root bridge, the ID of the bridge transmitting the BPDU, the

cost for that bridge to reach the root and the port used for forwarding in the direction of

the root. This port is referred to as the rootport. All other ports unless blockedare calleddesignated or forwardingports. The root ports and the designated ports constitute the

active links of the topology. A bridge with no root or designated ports is considered to be

a blocked bridge. Note that a blocked bridge does not participate in frame forwarding, butit does listen to all transmissions, monitoring all activity and ensuring the health of other

non blocked bridges. Not all ports on a bridge need to be either root or designated, somecan be blocked. Each LAN must have one and only designated bridge, which is the only

bridge with the a designated port on that LAN. Note that several bridges can have theirroot ports on a LAN, but only one can have a designated port.

2.1.2.1 Configuration BPDUs

These are the BPDUs used by the bridges to exchange information that will assist in the

determination of the spanning tree. Figure 10 shows the fields of a configuration BPDU.

The fields in red are the main fields used for creating the spanning tree.

Figure 10. Configuration BPDUs

time since root sent a

message on

which this message is based

Destination

MAC address

Source MAC

address

Configuration

Message

protocol identifier

version

message type

flags

root ID

Cost

bridge ID

port ID

message age

maximum age

hello time

forward delay

Set to 0 Set to 0 Set to 0

lowest bit is "topology change bit (TC bit)

ID of root Cost of the path from the

bridge sending this

message

internally assigned priority (Byte 1)

unqique port number (at this bridge)

(Byte 2)

ID of bridge sending this message

Time between

recalculations of the

spanning tree

(default: 15 secs)

Time betweenBPDUs from the root

(default: 1sec)


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Each bridge as a unique identifier: Bridge ID = . Abridge has several MAC addresses, one for each port, but only one ID, used to elect the

root. The lowest MAC address is usually used for the ID. Each port within a bridge has a

unique identifier (port ID).

The bridge with the lowest identifier is elected the root of the spanning tree and is

henceforth referred to as the root bridge (root ID in configuration BPDU). The root port

on each bridge identifies the next hop from that bridge to the root and is identified by theport ID in the BPDU. For each bridge, the cost of the min-cost path to the root is called

the root path cost. This cost is generally measured in hops (each LAN is a hop) to reach

the root, but it can be set to represent any other metric. For example a 100Mpbs LAN ismore desirable as a path than a 10Mbps LAN, and so the cost can be determined

accordingly, the 100Mbps LAN will have a lower value. The designated bridge on a LAN

provides the minimal cost path to the root for that LAN. If two bridges have the sameroot path cost, the algorithm selects the one with the highest priority. If the designated

bridge has two or more ports on the LAN, then the algorithm selects the port with thelowest identifier.

With the help of the BPDUs, bridges can:

Elect a single bridge as the root bridge.

Calculate the distance of the shortest path to the root bridge

Each LAN can determine a designated bridge, which is the bridge closest to theroot

The designated bridge will forward packets towards the root bridge.

Each bridge can determine a root port, the port that gives the best path to the root.

Select ports to be included in the spanning tree.

Below we describe the steps used to elect the root, determine the designated bridges andcalculate the minimum cost to the root.

2.1.2.2 Steps of the Spanning Tree Algorithm

Steps of the spanning tree algorithm:

Determine the root bridge

Determine the root port on all other bridges

Determine the designated port on each LANTo achieve the above, each bridge is sending out BPDUs that contain the root ID (whatthe bridge considers to be the root, initially always set to the bridges ID), root path cost

(current path cost to what is considered by the bridge to be the root, initially set to 0 asit assumes it is the root), bridge ID (own ID), port ID (port to be used to reach root).


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Figure 11 Main fields of configuration BPDU to calculate the Spanning Tree

Figure 12 Initial settings of the fields in a configuration BPDU

Initially, all bridges assume they are the root bridge. Each bridge B floods all its ports

with a configuration BPDU as shown in Figure 12 on its connected LANs. It identifiesitself, sets the root ID to itself and the cost is 0. Each bridge receives configuration

BPDUs from its neighbors and compares the values in the these three fields with those in

its own transmitted BPDU. It the updates its BPDU accordingly, the root bridge is the

smallest received root ID that has been received so far (whenever a smaller ID arrives,the root is updated) and it increments the root path cost by the cost of the link connecting

the bridge to the neighbor from which it received the BPDU with the lowest root ID.

Bridge Bs root port is the port from which B received the lowest cost path to the root R.With the new values of R, cost and root port, it can update its BPDU and flood that

information to all neighbors but the one it received the lowest root ID from. All ports

from which it received a BPDU with a higher root ID it will designate as its designatedports and assume itself to be the designated bridge for those LANs (unless two ports are

connected to the same LAN4, in that case, it will pick the one with the lowest ID) and

block the other.

Figure 13 Updated fields of BPDU

4A bridge will only know that if it receives to identical BPDUs.

root IDroot ID costcost bridge ID/port IDbridge ID port ID

BB 00 BB

RR CostCost BB

Bridge BPort A Port C

Port x

Port B


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Figure 14. Bridge Bs connected ports

For example, in Figure 14, if the BPDU with the lowest root ID was received from port x,

then bridge B will assume it is the designated bridge on the LANs attached to ports A,B,

and C.

To summarize our discussion on root and port selection, we can order BPDU messages

with the following ordering relation


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Figure 15 Mesh topology with a tree overlay. D identifies designated port, and R rootport

3 Tools and Utilities

3.1 Configuring a PC as a Bridge using the gbrctl utility

gbrctlis a GNOME utility to configure Ethernet bridging on Linux PCs. The

following screenshots illustrated the features and configuration procedure ofgbrctl.

To start the utility, type gbrctl at a shell terminal and the main window

appears:

LAN 2

Bridge 5

LAN 5

LAN 3

LAN 1

LAN 4

Bridge 4

Bridge 2Bridge 1

d

Bridge 3

DD

D

R

DR

R

R

D


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Figure 1: Main Window

To begin the configuration of Ethernet bridging, click onAdd Bridge, andenter the bridge name, such as Bridge1in the prompt that appears:

Figure 2: Prompt to Add New Bridge


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To configure gbrtclso that Ethernet interfaces of the Linux PC participate as

interfaces of the LAN switch, select Bridge1, and click onEdit Bridge. This will

bring up the Bridge Configuration Window:

Figure 3: Bridge Configuration Window Interfaces

Click on theInterfaces tab, and then the Add interfaces button. Type the name

of the interface to be added, e.g., eth0 oreth1.

Under the Settings tab, one may enable or disable the Spanning Tree Protocol(STP) by toggling the button next to the STP parameter:


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Figure 4: Bridge Configuration Window Settings

An important field under this tab is theBridge Priority. Probably due to aimplementation error,gbrctlhas an unconventional way to entering and

displaying the value of this field. By default,gbrctlsets theBridge Priority to

8000 (hex), and sets it to 0001 (hex) when SPT is disabled and then enabled. Toassign the bridge priority, enter the appropriate input value given in Table 1 below.

Input Value Value Displayed in GUI Actual Priority Value

8 0000 0

16 0008 8

32 0010 16

64 0020 32

128 0000 0

256 0040 64

512 0080 128

1024 0100 256

2048 0000 0

Table 1: Valid Bridge Priority Values forgbrctl


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To display the content of the forwarding table, click on theMACs tab and click the

Refresh MAC listbutton at the bottom:

Thegbrctltool does not have an explicit way to delete the forwarding table. In

order to delete the entries from the forwarding table, one may set the age of theforwarding table entries to a small value as follows:


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Select Bridge1 and click on Settings

Set Ageing to 0

Once the entries are deleted, set the Ageing entry to the original value (default is

300 seconds.)

3.2 Configuring a Cisco Router as a LAN SwitchA Cisco router can be operated as a LAN switch by turning off the routing functions withthe no ip routing command and enabling the bridging function. Table 1 below describes

the commands that need to be used. When enabling bridging, you also have to choose the

routing protocol, in our case we will be using the IEEE standard which refers to the

Spanning Tree algorithm. By choosing a priority, you can determine which bridge gets tobe the root. If all the bridges have the same priority, then the MAC address will be used

for root selection.

Router> enablePassword: rootroot

Enter the privileged EXEC mode. In thisstate you can read configuration files,

reboot, etc.

Router1# configureterminalRouter1(config)#

Enter a configuration mode. In theconfiguration mode, you can do varioussystem-related tasks, for example, assigningIP address, setting the protocol to support,etc.


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Router1(config)# no ip routing Disable IP routing. This tells the CiscoRouter to stop acting as a router.

Router1(config)# bridge 1 protocol ieee Assigns the IEEE Spanning Tree Protocol(STP) to bridge group 1. A bridge-group isa number between 1 and 9, which is chosen

to refer to a set of bridge interfaces.Router1(config)# bridge 1 priority 128 Define a priority for bridge group #1. Priority

is used when electing the root bridge in theSpanning Tree Algorithm.

Table 1 Enabling Bridging on a Cisco Router

Note: the bridge group number (e.g. bridge 1) is only relevant internal to eachrouter. If a router has at least 18 interfaces, you can create up to 9 bridge groups (9

pairs of interfaces). Since the Cisco routers in this lab only have 2 interfaces, we

will assign bridge group 1 to all the routers.

The above steps set up Router1 as a LAN switch. Now, each interface of the router has to be

individually configured to participate in LAN switching. In Table 2 we show the steps how toconfigure the interface Ethernet0 on Router1:

Router1(config)# interface eth0/0 Enter the interface configuration mode for interfaceEthernet0/0. This is used when configuringparameters specific to that interface.

Router1(config-if)# no mop enabledRouter1(config-if)# no mop sysid

These commands disable the MaintenanceOperation Protocol (MOP) in DEC networks. Bydefault, it is disabled on each interface. Thesecommands are applicable only on Cisco 25xxrouters, and are not available on Cisco 16xx or 36xxrouters.

Router1(config)# no cdp runRouter1(config-if)# no cdp enable

Disable the Cisco Discovery Protocol (CDP). Bydefault, it is enabled on each interface. When CDPis disabled, another type of device/networkmanagement protocol appears and transmitsloopback packets, which have identical source anddestination MAC addresses, and do not interferewith the experiments.

Router1(config-if)# bridge-group 1 Assigns this network interface to bridge-group 1.Frames are forwarded only between interfaces inthe same group within a bridge. In this lab, allinterfaces should belong to the same group.

Router1(config-if)# bridge-group 1 Assigns this network interface to bridge-group 1, but


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spanning-disabled disables the Spanning Tree Algorithm (SPT).

Router1(config-if)# no shutdown Activates the interface.

Router1(config-if)# end Returns to privileged EXEC mode.

Table 2. Setting the Router Interface

Once a Cisco router is configured as a LAN switch, the following commands are used todisplay the current status of the LAN switch:

Router1# show bridge Displays the entries of the forwarding table.

Router1# show spanning-tree Displays the spanning-tree topology informationknown to this bridge.

Router1# show interface Displays statistics of all interfaces, including theMAC addresses of all interfaces.

Table 3. Displaying the Bridge Status

The following steps are used to reset the state of a bridge at the beginning of a newexercise:

Router1# clear bridge Removes all entries from the forwarding table.

Router1# clear arp-cache Clears the ARP table.

Table 4. Resetting a Bridge

References:

http://home.planet.nl/~kristian/gbrctl.html

ch5 lan switching--it classroom

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